1. Field of the Invention
The present invention relates to a humidifying apparatus. More particularly, the present invention is concerned with a humidifying apparatus comprising:
a pleated functional element comprising a pleated structure and, secured to the pleated structure around a periphery thereof, a reinforcing frame, wherein the pleated structure is comprised of a humidifying membrane and, superimposed on at least one surface thereof, a gas-permeable reinforcing material layer, and
a dry-side channel and a wet-side channel which are, respectively, provided on opposite sides of the pleated functional element, wherein each of the dry-side channel and the wet-side channel has at least one pair of a gas-intake and a gas-outlet,
the humidifying apparatus having a first pressure-buffering means between the gas-intake and an outside conduit connected thereto and a second pressure-buffering means between the gas-outlet and an outside conduit connected thereto,
wherein the humidifying membrane divides the internal space of the pleated functional element into spaces which form a part or whole of the dry-side channel and a part or whole of the wet-side channel, respectively.
The humidifying apparatus of the present invention has excellent properties with respect to humidifying performance, smallness of pressure loss, volume efficiency and durability and can be used to perform efficient humidifying of various gasses. Therefore, the humidifying apparatus of the present invention can be suitably used for supplying moisture to a fuel cell.
2. Prior Art
A fuel cell is one type of an electric generator, in which a fuel, such as hydrogen or methanol, is subjected to electrochemical oxidation, to thereby generate an electrical energy. In recent years, fuel cells are attracting attention as a clean source of energy. In accordance with the type of electrolyte employed, fuel cells are classified into the phosphoric acid type, the molten carbonate type, the solid oxide type and the solid polymer electrolyte type. Of these types, the solid polymer electrolyte type is especially advantageous not in that a standard operating temperature is as low as 100° C. or less, but also in that a high energy density can be obtained. Therefore, fuel cells of the solid polymer electrolyte type are expected to give a wide variety of applications as power sources for use in automobiles and the like.
A solid polymer electrolyte type fuel cell is basically comprised of an ion exchange membrane and a pair of gas-diffusion electrodes which are, respectively, attached to the opposite surfaces of the exchange membrane. In operation, the two gas-diffusion electrodes are connected to each other through a load circuit positioned in the outside of the ion exchange membrane, and hydrogen and oxygen are, respectively, supplied to one and the other of the two gas-diffusion electrodes, thereby performing electric generation. More specifically, during the operation of the fuel cell, protons and electrons are generated at the hydrogen side electrode, and the generated protons moves through the ion exchange membrane toward the oxygen side electrode. When the protons reache the oxygen side electrode, they react with oxygen to form water. On the other hand, the generated electrons are led through the lead line to the external load circuit where the electric energy is taken out, and whereupon the electrons are further led through the lead line to the oxygen side electrode where the electrons contribute to the progress of the above-mentioned water forming reaction.
As a material for the ion exchange membrane, there are widely used fluorine-containing ion exchange resins by virtue of their high chemical stability. Of fluorine-containing ion exchange resins, there is widely used Nafion™ (manufactured and sold by E.I. duPont de Nemours & Company Inc.). “Nafion” is a perfluorinated polymer comprised of a perfluorocarbon main chain and pendant chains having a terminal sulfonic acid group. As well known in the art, for a fluorine-containing ion exchange resin to exhibit a high ion conductivity, it is necessary that the fluorine-containing ion exchange resin be well swollen with water. Therefore, in the case of applications in mobile machines (in which water supply is limited), specifically fuel cells installed in automobiles, how to ensure water supply to fuel cells is an important task.
As described above, in a fuel cell, water is formed by the reaction which occurs at the oxygen side electrode thereof. Therefore, if the water vapor contained in the exhaust gas discharged from the oxygen side electrode can be recovered and used to humidify a gas supplied to the oxygen side electrode or the hydrogen side electrode, self-sufficiency in water supply can be ensured, thereby removing the need for using a water tank or the like. In the present invention, the term “humidifying apparatus” means an apparatus in which a water vapor-permeable humidifying membrane is disposed to partition a space into two sides, wherein, in operation, water vapor contained in a gas present on one side of the humidifying membrane is used to humidify another gas present on the other side of the humidifying membrane. The humidifying apparatus for a fuel cell installed in an automobile is required to have the following properties:    1) Humidifying performance: the ability to provide moisture sufficient for the operation of a fuel cell.    2) Smallness of pressure loss: the pressure loss is satisfactorily small so as not to place a heavy burden on a compressor.    3) Volume efficiency    4) Durability
With respect to the prior art of the humidifying apparatus, for example, Unexamined Japanese Patent Application Laid-Open Specification No. Hei 11-354142 discloses a humidifying apparatus comprising a plurality of semi-permeable membranes (water vapor permeable membranes) which are stacked in the same direction as that of a stack of fuel cells. By this technique it is possible to humidify a gas supplied to the oxygen side electrode or the hydrogen side electrode; however, this technique has problem in that a considerably large number of semi-permeable membranes are required to be used in order to provide a membrane surface area which is necessary to effect a satisfactory level of humidifying, so that the increase in the number of semi-permeable membranes results in disadvantages not only in that there is increased the number of sealing portions of semi-permeable membranes, leading to an increase in the production cost, but also in that the number of gas passages (separators) for the semi-permeable membranes is increased, leading to a lowering of volume efficiency.
Unexamined Japanese Patent Application Laid-Open Specification No. Hei 8-273687 discloses a humidifying apparatus characterized in that it employs water vapor permeable membranes which are hollow fiber membranes. This technique is advantageous in that, because of the use of a hollow fiber, there is no need for the use of a separator (which is necessary in the above-mentioned Unexamined Japanese Patent Application Laid-Open Specification No. Hei 11-354142), thereby improving the volume efficiency. However, this technique has a problem in that the flow of gas is likely to be uneven due to the occurrence of an uneven positioning of hollow fibers, thus rendering it impossible to obtain a satisfactory humidifying. Further, since both of the inside and outside of a hollow fiber are exposed to a high speed gas stream, the hollow fiber receives internal and external vibrational forces, resulting in vigorous flattering of the hollow fiber. Such hollow fibers flattering vigorously undergo mutual friction and, hence, are likely to be abraded or broken. In addition, both fixed ends of each hollow fiber flattering vigorously sustain large stress and, hence, are likely to be broken.
Unexamined Japanese Patent Application Laid-Open Specification No. 2002-252012 discloses a humidifying apparatus which comprises a casing and, disposed therein, a cylindrical pleated element made of a water vapor permeable membrane, wherein the cylindrical pleated element is obtained by a method in which a pleated, water vapor permeable membrane is rolled into a cylindrical form, whereupon the both ends of the membrane which have come to meet each other by the rolling are gas-tightly sealed/connected to each other, and the both ends of the resultant cylindrical form are, respectively, gas-tightly sealed by connecting thereto doughnut-shaped terminal plates. This technique is advantageous in that, by virtue of the use of a pleated membrane, this technique is free from the problems of an increased cost and a lowering of volume efficiency (as encountered in the case of the technique of the above-mentioned Unexamined Japanese Patent Application Laid-Open Specification No. Hei 11-354142) and from the problem of a low durability (as encountered in the case of the technique of the above-mentioned Unexamined Japanese Patent Application Laid-Open Specification No. Hei 8-273687). However, this technique has a problem in that, as seen from the descriptions of FIG. 3 and paragraph 30 of this patent document, the above-mentioned cylindrical pleated element used in this technique generally has a hollow (dead space) having a diameter which is as large as about a half of the outer diameter of the cylindrical pleated element, so that the volume efficiency is still unsatisfactory. Further, this technique has also a problem in that, as seen from the description of FIG. 2 of this patent document, gas inlet 211 and gas outlet 212 are disposed directly above the surface of the cylindrical pleated element, so that most of the gas flows are directly blown onto the local surfaces of the cylindrical pleated element, thus rendering it difficult to effect even distribution of the gas flows onto the entire surface of the water vapor permeable membrane.
As described hereinabove, the conventional humidifying apparatuses have many problems to be solved, and there has not yet been developed a satisfactory humidifying apparatus.